1.
INTRODUCTION
A heat pipe is a simple device that can quickly transfer heat from one
point to another. They are often referred to as the "superconductors"
of heat as they possess an extra ordinary heat transfer capacity & rate
with almost no heat loss.
The development of the heat pipe originally started with Angier March
Perkins who worked initially with the concept of the working fluid only in one
phase (he took out a patent in 1839 on the hermetic tube boiler which works on
this principle). Jacob Perkins (descendant of Angier March) patented the
Perkins Tube in 1936 and they became widespread for use in locomotive boilers
and baking ovens. The Perkins Tube was a system in which a long and twisted
tube passed over an evaporator and a condenser, which caused the water within
the tube to operate in two phases. Although these early designs for heat
transfer systems relied on gravity to return the liquid to the evaporator
(later called a thermosyphon), the Perkins Tube was the jumping off point for
the development of the modern heat pipe. The concept of the modern heat pipe,
which relied on a wicking system to transport the liquid against gravity and up
to the condenser, was put forward by R.S. Gaugler of the General Motors
Corporation. According to his patent in 1944, Gaugler described how his heat
pipe would be applied to refrigeration systems. Heat pipe research became
popular after that and many industries and labs including Los Alamos, RCA, the
Joint Nuclear Research Centre in Italy
2. DESIGN
CONSIDERATIONS
The three basic components of a heat pipe are :
1. The
container.
2. The
working fluid.
3. The
wick or capillary structure.
2.1.
CONTAINER
The function of the container is to isolate the working fluid from the
outside environment. It has to
therefore be leak-proof, maintain the pressure differential across its walls,
and enable transfer of heat to take place from and into the working fluid.
Selection of the container material depends on many
factors. These are as follows:
- Compatibility (both with working fluid and external environment)
- Strength to weight ratio
- Thermal conductivity
- Ease of fabrication, including welding, machineability and
ductility
- Porosity
- Wettability
Most
of the above are self-explanatory. A high strength to weight ratio is more
important in spacecraft applications. The material should be non-porous to
prevent the diffusion of vapor. A high thermal conductivity ensures minimum
temperature drop between the heat source and the wick.
2.2. WORKING FLUID
A
first consideration in the identification of a suitable working fluid is the
operating vapour temperature range. Within the approximate temperature band,
several possible working fluids may exist, and a variety of characteristics
must be
examined
in order to determine the most acceptable of these fluids for the application
considered.
The
prime requirements are:
- compatibility with wick and wall materials
- good thermal stability
- wettability of wick and wall materials
- vapor pressure not too high or low over the
operating temperature range
- high latent heat
- high thermal conductivity
- low liquid and vapor viscosities
- high surface tension
- acceptable freezing or pour point
The
selection of the working fluid must also be based on thermodynamic
considerations which are concerned with the various limitations to heat flow
occurring within the heat pipe like, viscous, sonic, capillary, entrainment and
nucleate boiling levels.
In
heat pipe design, a high value of surface tension is desirable in order to
enable the heat pipe to operate against gravity and to generate a high
capillary driving force. In addition to high surface tension, it is necessary
for the working fluid to wet the wick and the container material i.e. contact
angle should be zero or very small. The vapor pressure over the operating
temperature range must be sufficiently great to avoid high vapor velocities,
which tend to setup large temperature gradient and cause flow instabilities.
A high latent heat of
vaporization is desirable in order to transfer large amounts of heat with
minimum fluid flow, and hence to maintain low pressure drops within the heat
pipe. The thermal conductivity of the working fluid should preferably be high
in order to minimize the radial temperature gradient and to reduce the
possibility of nucleate boiling at the wick or wall surface. The resistance to
fluid flow will be minimized by choosing fluids with low values of vapor and
liquid viscosities. Tabulated below are a few mediums with their useful ranges
of temperature.
Table 2.1 : TABLE OF A FEW MEDIUMS WITH THEIR
USEFUL RANGES OF TEMPERATURES
|
MEDIUM
|
MELTING PT.
( C )
|
BOILING PT. AT
ATM. TEMP. ( C )
|
USEFUL RANGE
( C )
|
|
Helium
Nitrogen Ammonia Acetone Methanol Flutec PP2 Ethanol Water Toluene Mercury Sodium Lithium Silver |
-271
-210 -78 -95 -98 -50 -112 0 -95 -39 98 179 960 |
-261
-196 -33 57 64 76 78 100 110 361 892 1340 2212 |
-271 to -269
-203 to -160 -60 to 100 0 to 120 10 to 130 10 to 160 0 to 130 30 to 200 50 to 200 250 to 650 600 to 1200 1000 to 1800 1800 to 2300 |
2.3. WICK OR CAPILLARY STRUCTURE
It
is a porous structure made of materials like steel, alumunium, nickel or copper
in various ranges of pore sizes. They are fabricated using metal foams, and
more particularly felts, the latter being more frequently used. By varying the
pressure on the felt during assembly, various pore sizes can be produced. By
incorporating removable metal mandrels, an arterial structure can also be
molded in the felt.
Fibrous
materials, like ceramics, have also been used widely. They generally have
smaller pores. The main disadvantage of ceramic fibres is that, they have
little stiffness and usually require a continuos support by a metal mesh. Thus
while the fibre itself may be chemically compatible with the working fluids,
the supporting materials may cause problems. More recently, interest has turned
to carbon fibres as a wick material. Carbon fibre filaments have many fine
longitudinal grooves on their surface, have high capillary pressures and are
chemically stable. A number of heat pipes that have been successfully
constructed using carbon fibre wicks seem to show a greater heat transport
capability.
The
prime purpose of the wick is to generate capillary pressure to transport the
working fluid from the condenser to the evaporator. It must also be able to
distribute the liquid around the evaporator section to any area where heat is
likely to be received by the heat pipe. Often these two functions require wicks
of different forms. The selection of the wick for a heat pipe depends on many
factors, several of which are closely linked to the properties of the working
fluid.
The
maximum capillary head generated by a wick increases with decrease in pore
size. The wick permeability increases with increasing pore size. Another
feature of the wick, which must be optimized, is its thickness. The heat
transport capability of the heat pipe is raised by increasing the wick
thickness. The overall thermal resistance at the evaporator also depends on the
conductivity of the working fluid in the wick. Other necessary properties of
the wick are compatibility with the working fluid and wettability.
The most common types of wicks that are used
are as follows:
Sintered Powder :
This
process will provide high power handling, low temperature gradients and high
capillary forces for anti-gravity applications. Very tight bends in the heat
pipe can be achieved with this type of structure.
Grooved Tube :
The
small capillary driving force generated by the axial grooves is adequate for
low power heat pipes when operated horizontally, or with gravity assistance.
The tube can be readily bent. When used in conjunction with screen mesh the
performance can be considerably enhanced.
Screen Mesh :
This
type of wick is used in the majority of the products and provides readily
variable characteristics in terms of power transport and orientation
sensitivity, according to the number of layers and mesh counts used.
3. WORKING
A
metal cylinder is sealed with a fluid within it creating a closed system. One
end of the tube is heated and the other is cooled. The heat source (the
evaporator) causes the fluid to boil and turn to vapor (this is absorbing
energy as heat). When that happens, the liquid picks up the latent heat of
vaporization. The gas, which then has a higher pressure, moves inside the
sealed container to a colder location where it condenses. Once the vapor
reaches the cold end of the tube (the condenser), the fluid changes phase again
from vapor back to a liquid. Thus, the gas gives up the latent heat of
vaporization and moves heat from the input to the output end of the heat pipe.
This liquid returns to the hot (evaporator) end by means of a wick so that the
liquid can
repeat
the process. This process is capable of transporting heat from a hot region to
a colder region. It requires no addition of external energy
Heat
pipes have an effective thermal conductivity many thousands of times that of
copper. The heat transfer or transport capacity of a heat pipe is specified by
its “ Axial Power Rating” (APR ).
It is the energy moving axially along the pipe. The larger the heat pipe
diameter, greater is the APR .
Similarly, longer the heat pipe lesser is the APR .
Heat pipes can be built in almost any size and shape.
4.
SPECIFIC TYPES OF HEAT PIPES
4.1. FLAT PIPES
Flat
heat pipes are just that; the orientation of the wick structure is designed so
that the liquid is more evenly distributed to the top and the bottom of the
plate.
The wick structure in a flat plate is a sintered metal; it is a metal
powder that has been molded and heated until the metal has fused, creating a
structurally stable metal with small pores within. Flat heat pipes produce a
surface that has a relatively uniform temperature distribution and large
surface area. These would be useful in the case where one needs to radiate heat
uniformly instead of from a point source. The use of flat plates as wall
components could be one possible application for heat pipe technology in the
house.
4.2. THERMAL SWITCHES
Thermal switches in a heat pipe serve to prevent the pipe from working
in certain cases. This can be accomplished by introducing a blockage, made
possible in a variety of different ways. Methods would include freezing the
fluid, placing a magnetically operated vane within the pipe which would block
the vapor flow, or using a physical displacement block (which controls the
amount of fluid in the reservoir and in the heat pipe by blocking the fluid
from being transported by the wick).
4.3. THERMAL DIODES
Another possible way to stop or control the heat transfer within the
pipe would be by limiting the acting surface of the condenser by using an inert
gas (this is the principle also behind variable conductance heat pipes).
Thermal diodes allow the heat pipe to only work in one direction. In one
example of a heat diode, if the location of the condenser and evaporator
switch, the liquid becomes trapped in a reservoir whose wicks are not connected
to the rest of the pipe. This makes it so that the liquid will not be able to
travel down the length of the heat pipe until the condenser and evaporator
switch again to heat the liquid to the gaseous phase so it can flow down the
pipe once more.
Another example of a thermal diode is when there is
excess liquid in a reservoir within the heat pipe. When the evaporator and
condenser are switched, the liquid in the reservoir becomes a vapor and
condenses on the condenser. This large amount of fluid prevents any vapor from
condensing at the other end of the heat pipe and therefore will only allow heat
transfer in one direction.
5. APPLICATIONS
Heat pipe has been, and is currently being, studied
for a variety of applications, covering almost the entire spectrum of
temperatures encountered in heat transfer processes. Heat pipes are used in a
wide range of products like air-conditioners, refrigerators, heat exchangers,
transistors, capacitors, etc. Heat pipes are also used in laptops to reduce the
working temperature for better efficiency. Their application in the field of
cryogenics is very significant, especially in the development of space
technology. We shall now discuss a brief account of the various applications of
heat pipe technology
5.1. SPACE TECHNOLOGY
The
use of heat pipes has been mainly limited to this field of science until
recently, due to cost effectiveness and complex wick construction of heat
pipes. There are several applications of heat pipes in this field like
- Spacecraft temperature equalization
- Component cooling, temperature control and
radiator design in satellites.
- Other applications include moderator cooling,
removal of heat from the reactor at emitter temperature and elimination of
troublesome thermal gradients along the emitter and collector in
spacecrafts.
5.2HEAT
PIPES FOR DEHUMIDIFICATION AND AIR CONDITIONING
In
an air conditioning system, the colder the air as it passes over the cooling
coil (evaporator), the more the moisture is condensed out. The heat pipe is
designed to have one section in the warm incoming stream and the other in the
cold outgoing stream. By transferring heat from the warm return air to the cold
supply air, the heat pipes create the double effect of pre-cooling the air
before it goes to the evaporator and then re-heating it immediately.
Activated
by temperature difference and therefore consuming no energy, the heat pipe, due
to its pre-cooling effect, allows the evaporator coil to operate at a lower
temperature, increasing the moisture removal capability of the air conditioning
system by 50-100%. With lower relative humidity, indoor comfort can be achieved
at higher thermostat settings, which results in net energy savings. Generally,
for each 1 F rise in thermostat setting, there is a 7% savings in electricity
cost. In addition, the pre-cooling effect of the heat pipe allows the use of a
smaller compressor.
5.3. LAPTOP HEAT PIPE SOLUTION
Heat
pipe technology originally used for space applications has been applied it to
laptop computer cooling. It is an ideal, cost effective solution. Its light
weight (generally less than 40 grams), small, compact profile, and its passive
operation, allow it to meet the demanding requirements of laptops.
For
an 8 watt CPU with an environmental temperature no greater than 40°C it
provides a 6.25°C/watt thermal resistance, allowing the processor to run at
full speed under any environmental condition by keeping the case temperature at
90°C or less.
One
end of the heat pipe is attached to the processor with a thin, clip-on mounting
plate. The other is attached to the heat sink, in this case, a specially
designed keyboard RF shield. This approach uses existing parts to minimize
weight and complexity. The heat pipe could also be attached to other physical
components suitable as a heat sink to dissipate heat.
Because
there are no moving parts, there is no maintenance and nothing to break. Some
are concerned about the possibility of the fluid leaking from the heat pipe
into the electronics. The amount of fluid in a heat pipe of this diameter is
less than 1cc. In a properly designed heat pipe, the water is totally contained
within the capillary wick structure and is at less than 1 atmosphere of
pressure. If the integrity of the heat pipe vessel were ever compromised, air
would leak into the heat pipe instead of the water leaking out. Then the fluid
would slowly vaporize as it reaches its atmospheric boiling point. A heat
pipe’s MTTF is estimated to be over 100,000 hours of use.
5.4. CPU WORK STATIONS
Heat
pipes have become widely used to cool the CPU's of computers due to the fact
that they can be manufactured at such a small scale. They act as heat sinks for
the processors and other components of computers that generate substantial
heat. The heat pipe solutions for thermal control at this level are a component
and overall systems requirement. Not only do the heat pipes take on a different
configuration with multiple heat pipes and cooling fins, but also airflow
becomes the critical design factor. Heat pipes designed to move 75 watts are
usually flat with fin stacks from three to six inches, in many cases with fins
mounted on each side of the CPU input pad
5.5. FLEXIBLE SOLUTIONS
Heat
pipes are manufactured in a multitude of sizes and shapes. Unusual application
geometry can be easily accommodated by the heat pipe’s versatility to be shaped
as a heat transport device. If some range of motion is required, heat pipes can
even be made of flexible mate Two of
the most common are:
Constant
Temperature: The heat pipe maintains a constant temperature or temperature
range.
Diode:
The heat pipe will allow heat transfer in only one direction.
5.6. MEGA FLATS
Flat
heat pipes are typically used for cooling printed circuit boards or for heat
leveling to produce an isothermal plane. Mega flats are several flat heat pipes
sandwiched together.
Some
of the flat heat pipes manufactured are:
XY
Mega Flats: Surface maintained within .01° F isothermal with concentrated load
centers.
6"
X 6" Mega Flat: Dissipated 850 watts from a printed circuit board.
Weight Reduction Mega Flats:
Standard - aluminum construction.
Lightweight - ½ the weight of
aluminum.
Very light weight - 1/3 the weight
of aluminum.
6. CONCLUSION
The cost
of heat pipes designed for laptop use is very competitive compared to other
alternatives. Cost is partially offset and justified by improved system
reliability and the increased life of cooler running electronics. Heat pipes,
in quantity, cost a few dollars each while an entire cooling system will cost
between $5 - $10 in production quantities, depending on the final design.
Standard design products are available to reduce cost even further. Heat pipe
manufacture has been a difficult area to compete in. Simple in concept, but
difficult to apply commercially, the heat pipe is a very elusive technology
& holds the key to the future of heat transfer & its allied
applications.
7.
REFERENCES
Andrews, J;
Akbarzadeh, A; Sauciue, I. : Heat Pipe
Technology, Pergammon, 1997.
Dunn, P.D.; Reay, D.A.: Heat Pipes, Pergammon, 1994.
www.heatpipe.com.
www.cheresources.com.
www.indek.com
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